As per ur last comment you are copying custom field value of product to Custom field of Quote Line Item(correct me if I am wrong).
In order to achieve this,u may have created process builder for Quote Line Item.
The formula that you have mentioned in description,it looks like you want to copy the value only when you create quote line item.
In the process builder condition you can create your formula like this:
In the above formula,I am copying the value of product description to Line Item description.I am checking if the Line item description field is blank and record is newly created.if formula sets to true then it copies the value of product description to line item description.
Below is the formula for immediate action.
After record is created and if I want to change the line item description then process builder will not trigger and I can update the description accordingly.
This is just an example that I have shown to you.you have to replace the fields with the custom field of yours in formula.
Let me know if it helpful to you.